Video data requires a lot of storage space to store or a wide bandwidth to transmit. Along with the growing high resolution and higher frame rates, the storage or transmission bandwidth requirements would be formidable if the video data is stored or transmitted in an uncompressed form. Therefore, video data is often stored or transmitted in a compressed format using video coding techniques. The coding efficiency has been substantially improved using newer video compression formats such as H.264/AVC and the emerging HEVC (High Efficiency Video Coding) standard.
FIG. 1 illustrates an exemplary adaptive Inter/Intra video coding system incorporating loop processing. For Inter-prediction, Motion Estimation (ME)/Motion Compensation (MC) 112 is used to provide prediction data based on video data from other picture or pictures. Switch 114 selects Intra Prediction 110 or Inter-prediction data and the selected prediction data is supplied to Adder 116 to form prediction errors, also called residues. The prediction error is then processed by Transform (T) 118 followed by Quantization (Q) 120. The transformed and quantized residues are then coded by Entropy Encoder 122 to be included in a video bitstream corresponding to the compressed video data. When an Inter-prediction mode is used, a reference picture or pictures have to be reconstructed at the encoder end as well. Consequently, the transformed and quantized residues are processed by Inverse Quantization (IQ) 124 and Inverse Transformation (IT) 126 to recover the residues. The residues are then added back to prediction data 136 at Reconstruction (REC) 128 to reconstruct video data. The reconstructed video data are stored in Reference Picture Buffer 134 and used for prediction of other frames. However, loop filter 130 (e.g. deblocking filter and/or sample adaptive offset, SAO) may be applied to the reconstructed video data before the video data are stored in the reference picture buffer.
FIG. 2 illustrates a system block diagram of a corresponding video decoder for the encoder system in FIG. 1. Since the encoder also contains a local decoder for reconstructing the video data, some decoder components are already used in the encoder except for the entropy decoder 210. Furthermore, only motion compensation 220 is required for the decoder side. The switch 146 selects Intra-prediction or Inter-prediction and the selected prediction data are supplied to reconstruction (REC) 128 to be combined with recovered residues. Besides performing entropy decoding on compressed residues, entropy decoding 210 is also responsible for entropy decoding of side information and provides the side information to respective blocks. For example, Intra mode information is provided to Intra-prediction 110, Inter mode information is provided to motion compensation 220, loop filter information is provided to loop filter 130 and residues are provided to inverse quantization 124. The residues are processed by IQ 124, IT 126 and subsequent reconstruction process to reconstruct the video data. Again, reconstructed video data from REC 128 undergo a series of processing including IQ 124 and IT 126 as shown in FIG. 2 and are subject to coding artefacts. The reconstructed video data are further processed by Loop filter 130.
In the High Efficiency Video Coding (HEVC) system, the fixed-size macroblock of H.264/AVC is replaced by a flexible block, named coding unit (CU). Pixels in the CU share the same coding parameters to improve coding efficiency. A CU may begin with a largest CU (LCU), which is also referred as coded tree unit (CTU) in HEVC. In addition to the concept of coding unit, the concept of prediction unit (PU) is also introduced in HEVC. Once the splitting of CU hierarchical tree is done, each leaf CU is further split into one or more prediction units (PUs) according to prediction type and PU partition. Furthermore, the basic unit for transform coding is square size named Transform Unit (TU). A Coding Group (CG) is defined as a set of 16 consecutive coefficients in scan order. For a given scan order, a CG corresponds to a 4×4 subblock. A syntax element coded sub_block_flag is signalled for each to indicate whether the subblock contains non-zero coefficients. If the subblock is significant as indicated by the corresponding flag, then the coefficient significant flag, sign flag, and absolute level of the subblock are further coded by up to five coefficient scan paths. Each coefficient scan path codes a syntax element within a CG, when necessary, as follows:                1) significant_coeff_flag: significance of a coefficient (zero/non-zero)        2) coeff_abs_level_greater1_flag: a flag indicating whether the absolute value of a coefficient level is greater than 1.        3) coeff_abs_level_greater2_flag: a flag indicating whether the absolute value of a coefficient level is greater than 2.        4) coeff_sign_flag: a sign of a significant coefficient (0: positive, 1: negative)        5) coeff_abs_level_remaining: the remaining value for absolute value of a coefficient level (if value is larger than that coded in previous passes).        
The bins in the first 3 passes are arithmetically coded in the regular mode (use context) and the bins in scan paths 4 and 5 are arithmetically coded in the bypass mode. Grouping bypass bins can increase the throughput of the entropy coder.
In the current HEVC standard, residuals in a TU is coded in the CG basis and the CGs are coded one by one according to CG scan path, where the CG scan path refers to the scan order for the CGs within a TU. Therefore, while the bypass bins within a CG are grouped together, the regular mode bins and bypass bins in a TU are still interleaved.
For each CG, depending on a criterion, coding the sign of the last non-zero coefficient is omitted when sign data hiding is applied. The sign value is derived by the parity of the sum of the levels of the CG, where an even parity corresponds to the positive sign and an odd parity corresponds to the negative sign. The criterion is the distance in scan order between the first and last non-zero coefficients. If the distance is larger than a threshold (i.e., 4 in HEVC), then sign data hiding is applied.
It is desirable to improve the coding efficiency especially for non-square transform units. Also, it is desirable to improve the throughput rate transform coefficient coding for coding groups.